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The more I read about making alcohol, the more I realize how little I know.

I stumbled onto something called the "Crabtree Effect", and it has turned my understanding of aerobic vs anaerobic fermentation upside down.

I am sure a lot of you have read about how yeast will reproduce in the presence of oxygen (aerobic fermentation) and that they will make ethanol in the absence of oxygen (anaerobic fermentation).

And I am sure that a lot of folks like me thought that all you had to do to get yeast to reproduce and build up a nice colony was throw some oxygen into the mix. But it's not nearly that simple.

You will have to google about the Crabtree Effect and read through a dozen different scientific documents to understand it.

Or you can read this article right here. This webpage seems to sum up the general gist of it about as well as anything.

It suddenly makes sense to me why my "yeast bomb" has such a long lag-phase. I have been throwing way too much sugar at the yeast and effectively suffocating them.

The more you go overboard with the sugar, the longer the lag-phase.

If you throw a cup of sugar at a gallon-sized yeast bomb, the yeast have to turn about 97% of that sugar into alcohol until they get the sugar content down below 9 grams of sugar per liter, then they are able to start taking up oxygen and reproducing.

That's the magic number: 9 g/l. That's almost nothing. That is about 1 tablespoon of sugar per liter (or quart). Anything more than that, and the yeast will make ethanol until the sugar content gets down below 9 g/l, no matter how much oxygen you throw at them.

And if a few yeast cells do manage to take up a little oxygen in that higher concentration of sugar and reproduce, the child-cells that they produce will never be as strong and healthy as the parent-cells. So they will be the first ones to croak at the first sign of trouble.

A little too much alcohol: weak child-cells croak. Temperature a little too high or low: weak child-cells croak. Acid out of range: weak child-cells croak.

I'm guessing the only yeast cells that actually make it to the end of most fermentations are those strong, healthy yeast cells that came from those first couple tablespoons that you started with.

Back to the yeast bomb.

So you go easy on the sugar in your yeast bomb, the yeast take up that tiny bit of sugar, they make some healthy new yeast cells. And then they sit there waiting for you to add more sugar.

You have to literally spoon feed the sugar to them if you want to increase the number of yeast in your yeast bomb in a timely fashion.

I don't think you could even use a hydrometer to tell you when to add more sugar. It's such a low sugar concentration, I doubt seriously we would be able to see the difference on a hydrometer.

I don't even think we could taste the difference because we're working with such a low sugar concentration.

The only way to make more yeast cells is to spoon-feed sugar at the right rate. Otherwise, we are just kicking the yeast over into anaerobic mode making ethanol until they get the sugar concentration down to where they feel like reproducing again.

I am going to have to do some experimenting with my next yeast inoculation (bomb) to see if the yeast give me any sort of indicator as to when I should add more sugar.

I just don't know what I should be looking for. Foam on top? CO2 bubbles? An increase in haziness?

Buccaneer Bob, I read the article and my understanding of the 'Crabtree effect' is that yeast in high sugar will be subjected to fermentation rather than aerobic respiration. This is partially true but that doesn't mean they aren't using O2. Actually the yeast are actively dividing and use the O2 to make natural unsaturated steroid alcohol called sterols. The yeast alter the rate of propagation as dictated by the state of the environment, such as insufficient O2, amino acids, or other raw materials needed to build new cells. As resources are used up, propagation rate decreases.

This brings me back to the birdwatchers recipe. If you read through the thread Tater starts it using actually another distiller called birdwatchers recipe. In it Tater states to stir the fermentation the first three days which introduce the oxygen. Rad later states pitch the yeast and let it go. I have used both methods and can't recollect a difference in either abv or speed of fermentation.

What you are suggesting is to add sugar as the yeast build. I would be interested in what you find out but if you post results you need to do it scientifically with controlled environments. My guess is since the current method works and unless you can improve one of the above stated outcomes or improve flavor folks here aren't going to want to put that kind of effort into their fermentation.

Another positive outcome to your study might be the result of solving the potential 'stuck fermentation' which would be a positive especially on new recipes that have not been tested.

Like the saying goes "just my 2 cents worth"
Bushman

Bushman wrote:This brings me back to the birdwatchers recipe. If you read through the thread Tater starts it using actually another distiller called birdwatchers recipe. In it Tater states to stir the fermentation the first three days which introduce the oxygen. Rad later states pitch the yeast and let it go. I have used both methods and can't recollect a difference in either abv or speed of fermentation.

So you're saying you couldn't see much difference between stirring vs no-stirring, huh? That seems to indicate that there probably is something to this Crabtree Effect, since introducing oxygen didn't seem to make much of a difference in how the wash ferments.

What you are suggesting is to add sugar as the yeast build.

Yeah. My angle is really focused on making a good quality yeast bomb (or inoculation) that could be dumped into a wash later. I think it would be way too much trouble to start with a fermenter full of water and a little bit of yeast, and then add whatever sugars to a full batch of wash over time.

I would be interested in what you find out but if you post results you need to do it scientifically with controlled environments.

I would love to do that, but I'm not sure how I would gauge how many yeasts are being created. The only reliable way that I can think of would be using a microscope, which really isn't in my budget right no. And then I wouldn't have any way to evaluate whether the child-yeasts are full-strength yeast or weaker.

My guess is since the current method works and unless you can improve one of the above stated outcomes or improve flavor folks here aren't going to want to put that kind of effort into their fermentation.

True.

Yeah, I'm still not sure what to make of it all or how best to proceed with the information I have been gathering on the Crabtree effect.

It does seem to explain some things that have been puzzling me, like the lag-time before my yeast bombs really start becoming active.

And the concept of the weak child-cells might explain a ton of things about:
A) why most yeast colonies tend to degrade over time -- because the weak child-cells croak early and the parent cells can only live so long before they croak too;
B) why we can have this thick layer of yeast sludge at the end of a ferment, indicating plenty of yeast being produced, but the ferment never really performs as well as expected -- again, the weak child-cells croak early, leaving us all of that sludge, and the parent-cells have to do the vast majority of the work from start of fermentation to finish; and
C) why some fermentations stick -- parent-cells essentially die of "old age" and child-cells aren't strong enough to continue the fermentation.

If you calculate the number of yeast cells needed for your wort and pitch that amount there's no worries about cell wall strength. Beer Smith's easy and fast but there's free apps out there too. I like the flavor of rum under pitched dry with baker's yeast though, pitching the "correct" amount doesn't produce the same taste complexity. All in what tastes good to you.

heartcut wrote:If you calculate the number of yeast cells needed for your wort and pitch that amount there's no worries about cell wall strength. Beer Smith's easy and fast but there's free apps out there too. I like the flavor of rum under pitched dry with baker's yeast though, pitching the "correct" amount doesn't produce the same taste complexity. All in what tastes good to you.

Good points.

Yeah, I have been noticing that my slower rum ferments seem to have a bit more flavor.

I have been thinking more about what heartcut was saying, and I think that maybe there are two yeast strategies that are both good.

Good Strategy #1: Throw a good batch of store-bought yeast at the wash and let those particular yeast cells do everything from start to finish with minimal exposure to oxygen. That way, all they do is make alcohol, and they don't waste any energy or resources trying to make child-cells.

Good Strategy #2: Build a good yeast bomb, but do it right with plenty of exposure to oxygen during the yeast bomb phase and sugar content somewhere below 9 g/l. This way, a good sized colony of healthy, strong yeast are created. Once pitched into the wash, oxygen exposure should be kept to a minimum so that the yeast that are created during the yeast bomb phase can focus their energy on tearing through the wash's sugars like gangbusters, rather than producing more yeast cells.

And then there's a bad strategy.

Bad Strategy: Put yeast in an environment with much more than 9 g/l of sugar and give them some oxygen, opening the door for them to multiply, but under unfavorable conditions. The parent-cells become weaker and weaker as they give up more and more of their "chi" (or "life energy") trying to create child-cells, and they end up dying young because of it. Then the scrawny child-cells that were created crap their pants and die at the first sign of trouble, leaving us with no yeast or nearly no yeast to complete the fermentation. So the fermentation either sticks, or it takes it takes much longer to complete than necessary.

Does that make sense?

So what you are saying is that adding O^2 is bad for the yeast in a high sugar ferment?!? All these stir plate starters that have fairly high sugar content and are taking in O^2 all day long an a stir plate is actually making a large but weak colony? I'm going for WTF, final answer!

I have never had a stuck ferment when throwing massive O^2 at the beginning of a ferment. In fact, most of my meads are really high in sugar at the beginning of the ferment and I typically will add O^2 the first and second day. I use a fish pump and SS air stone and drop it in for 30 min on day 1 and another 30 min on day two, np.

The only stuck ferment I have ever had was due to apple juice with preservatives so I 'll be sticking to my usual O^2 process.

I know what you're saying, Durace11. You could have knocked me over with a feather when I first read about this Crabtree Effect deal.

I started doing more research into it, and if I didn't know better, I would almost think that these scientists live in a completely different universe than the rest of us.

I have been thinking that most brewers/winemakers/distillers have been passing traditions down from generation to generation, and the traditional way of doing things works for most of us, so we all just continue to do it the traditional way.

Meanwhile, the scientists are looking at the Crabtree Effect phenomenon in a laboratory environment, and they are seeing some very good reasons for propagating yeast in a low-sugar environment before they are launched into a wort/must/wash.

But the trouble is, either the scientists are not doing a good enough job of evangelizing the benefits of doing it their way, or else they have tried to evangelize their method, but nobody is buying it.

Me, I'm thinking I am going to try the low-sugar method in my next few yeast bombs and see if I can see some sort of benefit to doing it the way that the scientists are recommending.

But if I don't see any real-world advantage to doing it the scientist-recommended way, I will probably go back to doing it the traditional brewer/winemaker/distiller way.

I virtually always do a low sugar start and add the sugar in stages. Sometimes 3 stages but usually just the two. I always did this with wine and with mead I am even more careful about watching the SG of the fermentation. I mostly ferment fruit and always ferment on the fruit pulp for a week before adding any further sugar.

My yeast gets activated and then established as a starter in a 1 gallon demijon with diluted wash before it goes into the fermenter.

The Arroyo patent for heavy rum also mentions this.

Thanks, myles. That's helpful knowing that you're doing it that way, and it's working for you.

I can't wait to do my next yeast bomb so I can try it for myself.